1. Field of the Invention
This invention relates to a fluorination catalyst and a fluorination process. More particularly, the invention relates to an improved fluorination catalyst for producing a hydrofluorocarbon (hereinafter referred to as "HFC") at a high yield and a process for efficiently producing the HFC with high productivity by bringing hydrogen fluoride into contact with a halogenated hydrocarbon in a gaseous phase by using the fluorination catalyst, when the HFC, which precludes the possibility of destruction of the ozone layer because it does not contain chlorine in the molecule thereof, particularly difluoromethane (hereinafter referred to as "HFC-32"), 1,1,1,2-tetrafluoroethane (hereinafter referred to as "HFC-134a") and pentafluoroethane (hereinafter referred to as "HFC-125"), is produced.
2. Description of the Related Art
Typical industrial production processes for an HFC, according to the prior art, include a process which brings a hydrogen-containing halogenated hydrocarbon into contact with HF and converts halogens such as chlorine, bromine and iodine into fluorine (in some cases, by using an unsaturated halogenated hydrocarbon as a raw material, reactions to add HF and convert halogens such as chlorine, bromine and iodine, into fluorine may simultaneously occur), and a process which brings a halogenated hydrocarbon into contact with H.sub.2 and exchanges the halogens, such as chlorine, bromine and iodine (although some fluorine atoms, in some cases) with H. Among these reactions, the fluorination reaction of a hydrogen-containing halogenated hydrocarbon with HF does not proceed smoothly in many cases, and the production quantity of the HFC depends greatly on the catalyst used.
A typical example of an unsmooth reaction is the synthesizing reaction of HFC-134a by fluorination of 1-chloro-2,2,2-trifluoroethane (hereinafter referred to as "HCFC-133a"). This reaction is an endothermic reaction which is thermodynamically disadvantageous. For this reason, the reaction is generally carried out by adding HF in an amount exceeding a stoichiometric amount to HCFC-133a and other reaction conditions (pressure, temperature, space velocity) are chosen to provide a significant conversion ratio of HCFC-133a. By way of example, Japanese Unexamined Patent Publication (Kokai) No. 55-27138 obtains HFC-134a at a yield of 32% using a compound obtained by treating CrF.sub.3.3H.sub.2 O with air as the catalyst under a reaction condition where the reaction pressure is atmospheric pressure, the reaction temperature is 400.degree. C., a molar ratio of HF to HCFC-133a (hereinafter called the molar ratio) is 8 and the space velocity (hereinafter abbreviated to SV) is 550 h.sup.-1. U.S. Pat. No. 4,922,037 obtains HFC-134a at a yield of 32% using a catalyst obtained by fluorinating CoCl.sub.2 /Al.sub.2 O.sub.3, under a reaction condition where the pressure is atmospheric pressure, the temperature is 410.degree. C., the molar ratio is 10 and the contact time is 30 seconds (SV 48 h.sup.-1). The reaction at such a low SV provides low productivity, and the reaction at the high temperature invites not only a heat energy loss but also the drop of selectivity. Furthermore, according to the studies of the present inventors, it invites a reduction in catalyst life. Accordingly, various studies have been made to attain higher activity of the catalyst and to prolong its service life. Japanese Unexamined Patent Publication (Kokai) No. 2-172933, for example, discloses that a catalyst comprising a halide or an oxide containing Cr and at least one element selected from the group consisting of Al, Mg, Ca, Ba, Sr, Fe, Ni, Co and Mn, and Cr, has high durability (life). EP 502605 teaches that a Cr-containing catalyst supporting Zn exhibits high activity. Further, European Patent Publication No. 516000-A1 describes that a catalyst comprising partially fluorinated Cr.sub.2 O.sub.3 supporting Ru and Pt has long life.
As a catalyst using a component other than Cr as the principal component, the Applicant of the present invention proposed in Japanese Unexamined Patent Publication (Kokai) No. 2-95438 a catalyst which contains an In compound supported on a support such as alumina, and is treated by HF. However, activity of this catalyst is lower than that of the catalysts using Cr as the principal component.
As described in the specification of Japanese Unexamined Patent Publication (Kokai) No. 4-346943, however, it has been clarified that when the fluorination reaction for HCFC-133a with HF is carried out using the Cr-containing or Al-containing catalyst according to the prior art, a new problem inherent to this reaction occurs, in that the reaction rate falls and productivity drops when the reaction pressure is elevated. In other words, even in the case of a catalyst which provides a high yield at atmospheric pressure, the yield drops remarkably because the conversion of HCFC-133a drops when the reaction pressure is elevated (to 10 kg/cm.sup.2 G (gauge pressure), for example), even though selectivity to HFC-134a can be somewhat improved. (Other conditions such as the reaction temperature, the molar ratio, SV converted to the standard state, etc, are kept the same for the purpose of comparison). This phenomenon can be likewise observed in the fluorination reaction of other hydrogen-containing halogenated hydrocarbons, though the degree is somewhat different.
When the reaction is carried out at an atmospheric pressure of about 1 kg/cm.sup.2 G in practical production equipment, additional equipment becomes necessary to reduce the pressure of the reactor, and this results in an undesirable increase in the cost. Further, performing the reaction at an elevated pressure provides higher selectivity and it can particularly restrict the production of unsaturated compounds, having high toxicity, as by-products. Accordingly, development of a catalyst which does not cause the reaction rate to fall even when the reaction pressure is elevated and further preferably, a catalyst which increases the reaction rate when the reaction pressure is elevated, has been desired.
Low activity of the catalyst, and a short service life, that have been problems in the prior art, are also improved by the present invention because they greatly contribute to the catalyst cost and productivity.